Vehicle testing often requires simulation of the loading effects encountered during road operation. This is accomplished with a dynamometer connected to the vehicle power train; usually a chassis roll dynamometer turned by the vehicle wheels. The dynamometer is controlled to provide appropriate energy absorption or input so that the speed versus time profile of the vehicle powertrain is the same as it would be in road operation for any accelerator pedal movement versus time profile; i.e. the vehicle drives the same on the roll of the dynamometer as on the road.
It is desirable for the dynamometer to be able to accommodate the speed and acceleration capabilities of a wide range of vehicles, ranging from small passenger cars to large trucks, and provide accurate speed profile simulation without mechanical changes in the dynamometer machinery, such as connecting mass-simulating flywheels. A DC dynamometer with motoring and absorbing power in the same range as the most powerful vehicles to be tested can provide the required simulation if controlled correctly. However, dynamometer controls often do not provide accurate speed profile simulation in response to changes of vehicle power, especially when the mass of the vehicle being simulated is a large multiple o fraction of the equivalent mass of the dynamometer machinery.
A simulation error usually occurs when the vehicle power changes. This error results from the inevitable time delay required to measure the change in acceleration rate produced by the vehicle power change, during which the dynamometer machinery speed changes differently than the vehicle's speed would change on the road. As the difference between the simulated vehicle mass and the dynamometer machinery rotational mass increases, this error also increases. The speed profile must be restored to road-equivalency quickly for accurate simulation.
As described in U.S. Pat. No. 4,327,578 to S. D'Angelo, issued May 4, 1982, entitled DYNAMOMETER, dynamometer control methods have been developed which provide most of the required simulation. However, these methods do not address the speed error developed during the first control cycle after a vehicle power change. The reason why these control methods do not address this problem may be that heretofore dynamometers have been limited to relatively small ratios of vehicle to dynamometer mass, for example a ratio of about 3 to 1, where the errors are minor for a fast control. However, for much larger ratios of vehicle to dynamometer mass, for example a ratio of about 30 to 1, the errors can become significant. Therefore, it would be desirable to have a dynamometer control method which includes correction for the speed error so as to provide a more accurate control of the dynamometer. cl SUMMARY OF THE INVENTION
A method of controlling a dynamometer having means for driving engagement with the drivetrain of a vehicle to be tested, a power absorption and motoring unit coupled with the driving engagement means for simulating road load and inertia forces, a torque transducer coupled to the power absorption and motoring unit for providing a force load signal and a speed transducer for providing a speed signal includes the steps of determining the torque and speed over short time intervals. The speed at which the driving engagement means should be moving is determined and compared with its actual speed. The torque of the power absorption and motoring unit is adjusted until the determined speed and actual speed agree. The adjustment in the torque includes not only changes in the torque necessary to keep the acceleration rate road-equivalent but also changes necessary to correct for speed errors.